The present invention relates to the in situ leaching of mineral values and more particularly relates to a method and apparatus for mixing a gaseous oxidant and a lixiviant at a downhole location for use in an in situ leach operation.
In a typical in situ leach operation, wells are completed into a leachable mineral-bearing formation and a lixiviant is flowed between wells to dissolve the mineral values into the lixiviant. The pregnant lixiviant is produced to the surface where it is treated to recover the mineral values from the lixiviant.
Many leachable mineral values, as they occur in their natural state in a formation, must be oxidized to a higher valence before they become soluble into a lixiviant. For example, uranium is normally present in a formation in the tetravalent state and must be oxidized to the hexavalent state to render it soluble in a suitable lixiviant, e.g., an aqueous carbonate solution. To oxidize uranium to its higher valence, it is customary to contact the uranium in the formation with an oxidant which may be injected directly into the deposit or which may be mixed into the leach solution and injected therewith.
Several oxidants have been proposed for this purpose, including gaseous oxidants such as air and oxygen. For example, in U.S. Pat. No. 3,708,206, oxygen is injected into a formation prior to or simultaneously with a lixiviant. In U.S. Pat. No. 3,713,698, air is injected through a production well to oxidize uranium values prior to injecting a lixiviant through an injection well. In both U.S. Pat. Nos. 3,640,579 and 3,860,289, oxygen is supplied through a tube to a downhole location where it is bubbled into a lixiviant before the lixiviant is injected into a formation.
With each of these types of injection schemes, excess quantities of oxygen are required to dissolve a sufficient amount of oxygen into the leach solution. For example, where oxygen is merely bubbled into the lixiviant downhole before the lixiviant enters the formation, experimentation suggests that a tenfold to fiftyfold excess of oxygen over the saturation requirement is needed to bring the leach solution to a level of 80% oxygen saturation, resulting in excessive oxygen costs. Of course, this excess oxygen could be collected, recompressed, and recycled; but the cost of doing this is equally as excessive.
Another approach to mixing oxygen and lixiviant downhole might be to use a mechanical agitator (beater) downhole. However, the high capital investment, along with operational and maintenance costs, makes such an approach impractical. Still another mixing approach is to inject oxygen through fine frits or spargers located downhole to form small bubbles in the lixiviant to effect a good mass transfer of oxygen to the lixiviant. However, based on known in situ leach conditions, precipitates present in normal leach operations will likely plug the frits quickly thereby severely restricting the necessary oxygen flow.
Therefore, for a method of mixing the gaseous oxidant and the lixiviant together at a downhole location to be economical and operationally functional, it should involve apparatus which (1) is not easily susceptible to plugging, either by materials carried in the lixiviant or by precipitates resulting from chemical reactions of the oxidant with materials in the lixiviant; (2) is effectively self-controlling, i.e., one not requiring complex controls at either the surface or downhole; (3) requires little or no excess oxidant to fully saturate the lixiviant; and (4) requires no additional energy sources such as power for motor-driven downhole mixers or the like. At the same time, the mixing method and apparatus should provide for a high rate of mass transfer of oxidant to the lixiviant, prevent substantial amounts of oxidant from bubbling to the surface, and restrict the amount of undissolved oxidant in the lixiviant before it enters the formation to be leached.